Vigorous means for the agitation of metals during solidification are known to eliminate dendritic structures and produce semisolid "slurry structured" materials with thixotropic characteristics. It is also known that the viscosities of such materials may be high enough to be handled as a soft solid. Re Rheocasting, Merton C. Flemings and Kenneth P. Young, McGraw-Hill Yearbook of Science and Technology, 1977-78. However, processes for producing shaped parts from such slurry structured materials, particularly on a continuous basis, present a number of problems. Such processes require a first step of reheating a slurry structured billet charge or "slug" to the appropriate fraction solid and then forming it while in a semisolid condition. A crucible has been considered essential as a means of containing the material and handling it from its heating through its forming cycle. The use of such crucibles is costly and cumbersome and furthermore creates process disadvantages such as material loss due to crucible adhesion, contamination from crucible degradation and untoward chilling from random contact with crucible side walls. Other problems are involved in the heating, transport and delivery of billets which are in a semisolid condition. Therefore, it has been found desirable to provide an apparatus and process for producing shaped metal parts from semisolid preforms, as such a process provides considerable manufacturing economy, especially if it does not require crucibles or other containing means and it is capable of operation on a continuous basis.
One approach has been the use of induction heating. Induction heating is a preferred choice for slug heating in SSM forming since it is clean, efficient, and can be tightly controlled via electrical input. A number of induction heating schemes have been devised and at least one is the subject of U.S. Pat. No. 4,569,218 issued Feb. 11, 1986, hereby incorporated by reference. Nearly all of these existing SSM heating systems utilize vertically oriented solenoid induction coils which either 1) raise and lower to sequentially capture slugs transported through the system on refractory pedestals, or 2) are stationary but through which the slugs are raised and lowered as they transport through the system.
However, such systems may be cumbersome with a number of moving parts. In addition, the persistent raising and lowering of either slugs or coils exposes the slugs to a variety of conditions which can pose problems of differential heat losses and consequent inconsistency of heating. Furthermore, since the slugs move both vertically and horizontally it is extremely difficult to provide protective atmosphere or heat shields.
Therefore, there is a recognized need for an improved method and apparatus for heating metal members to a semi-solid state, in order that the members may be formed as desired.